Taming the Instruction Bandwidth of Quantum Computers via Hardware-Managed Error Correction

A quantum computer consists of quantum bits (qubits) and a control processor that acts as an interface between the programmer and the qubits. As qubits are very sensitive to noise, they rely on continuous error correction to maintain the correct state. Current proposals rely on software-managed error correction and require large instruction bandwidth, which must scale in proportion to the number of qubits. While such a design may be reasonable for small-scale quantum computers, we show that instruction bandwidth tends to become a critical bottleneck for scaling quantum computers. In this paper, we show that 99.999% of the instructions in the instruction stream of a typical quantum workload stem from error correction. Using this observation, we propose QuEST (Quantum Error-Correction Substrate), an architecture that delegates the task of quantum error correction to the hardware. QuEST uses a dedicated programmable micro-coded engine to continuously replay the instruction stream associated with error correction. The instruction bandwidth requirement of QuEST scales in proportion to the number of active qubits (typically <; <; 0.1%) rather than the total number of qubits. We analyze the effectiveness of QuEST with area and thermal constraints and propose a scalable microarchitecture using typical Quantum Error Correction Code (QECC) execution patterns. Our evaluations show that QuEST reduces instruction bandwidth demand of several key workloads by ftve orders of magnitude while ensuring deterministic instruction delivery. Apart from error correction, we also observe a large instruction bandwidth requirement for fault tolerant quantum instructions (magic state distillation). We extend QuEST to manage these instructions in hardware and provide additional reduction in bandwidth. With QuEST, we reduce the total instruction bandwidth by eight orders of magnitude.